IK local rotation locking
In fact, the rotation locks are applied to the local axes. As long as the bone is oriented correctly, you will get the correct results. When creating multiple copies and rotating, you must make sure to adjust the roll of the bone (in Edit Mode) in the Bone properties tab to the correct direction.

To confirm the orientation of the bones, you can go to the Armature properties tab, and under Display, check Axes. Now when you look at your bones, the tip of it will display the directions. Keep in mind that the axes will display at the tip, so don't get confused by another bone connected to the base as it only applies to that bone.

From there, you can choose your IK locking settings for the bone according to what the bone's display shows.